Battery low voltage

I have problem with battery voltage failsafe. FS voltage is set to 14V but battery voltage is dropping under 14 volts in stable flight and even under 13v during takeoff(?). Is it just bad battery or am I flying bad setup?
Frame: Tarot 680 pro
Motors: RC timer HP 2814-810KV
Battery: Turnigy 2x 6000mah 4s 25-50c (old)

Here is battery voltage log. After takeoff short time hovering and then triggers failsafe. Same problem with one battery.

Post a link to the log. What is the take-off-weight (with battery) and what props?

Never shared a log. Is this working? Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.
Take-off-weight: 3200g
props: 12 inch (not sure about the pitch)

Thrust to weight looks good. You could try sag compensated voltage as the reference for the Failsafe. Or get a new battery.

You have a lot of default parameters values and obviously have not run the Initial Tuning Settings in Mission planner. The Tune is a not that great.You should really have Motor Thrust scaling configured:

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Thank you for your answer. I have done some modifications of those values (still not tested yet). But why there is options 6 53 and 6 42 in MOT_BAT_VOLT_MIN/MAX? I put 4.2 in MOT_BAT_VOLT_MAX and mission planner said that it is “out of range” is it still safe to keep it that way?

Use this spreadsheet, or the Initial Parameters section of MissionPlanner - you can just choose to update only the parameters you need to. Be sure to select the correct battery chemistry.
You should have something like:

Also the temptation is to lower those voltage thresholds (the BATT_ values) to allow arming or longer flight time - don’t give in to this temptation as it only makes the battery worse and increases risks.
Just get a new battery and look after it.

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Thank you! That was very useful Excel. Yes I understand that lower those values is not way to go :slight_smile: